The present invention relates to a multi-layer film or sheet material or, more particularly, to a multi-layer film or sheet material of high adhesive bonding strength between layers having excellent susceptibility to adhesive bonding and heat-sealability as well as excellent impact strength at low temperatures and rigidity and a laminated film or sheet material with the multi-layer film or sheet material as the sealing layer.
As is well known, films of polypropylene-based resins are widely used in a variety of applications by virtue of the excellent heat resistance, mechanical properties, workability and the like properties thereof. The fields of application of polypropylene-based films are, however, under certain limitations due to the insufficiently high heat-sealability and adhesive bondability thereof. Accordingly, proposals have been made to blend the polypropylene-based resin with a polyethylene, polyethylene-based copolymer, thermoplastic elastomer and the like although the miscibility or compatibility of these additive polymers with polypropylene is not always sufficiently high so that several problems are unavoidable that the resultant blended resin has decreased transparency and the excellent properties such as heat resistance, rigidity and the like inherently possessed by the polypropylene-based resins are sometimes lost.
Therefore, another proposal has been made of a coextruded composite film material composed of a polypropylene-based resin and a linear low-density ethylene .alpha.-olefin copolymer. Although the heat-sealability can be improved to some extent in the composite films of this type, such a composite film is defective not only in the insufficient adhesive bonding strength between layers but also in the poor adhesive bondability in lamination with a base film of different kind.
On the other hand, stretched films of polypropylene, polyamide, polyester and the like resins and aluminum foils are widely used as a base film for wrapping of, for example, various kinds of foods by virtue of the excellent heat resistance, moisture- and gas-impermeability and other properties but these materials are poor in the heat-sealability so that they are usually used as a component of a multi-layer film or sheet material with other heat-sealable films.
To describe the method for the preparation of such a multi-layer film or sheet material, the above mentioned base film and a heat-sealable film are bonded together to form a laminate by the so-called dry lamination using a polyurethane-type adhesive and the like or by the extrusion lamination using a low-density polyethylene film as the adhesive layer. When a polyethylene-based film is used as the heat-sealable film in this case, however, the excellent property of high heat resistance possessed by the base film may eventually be lost.
Accordingly, the most widely used heat-sealable film is an nonstretched film of polypropylene in the fields where high heat resistance, mechanical strength and rigidity are essential. Nonstretched polypropylene films used as the heat-sealable film, however, are poor in the adhesive bondability so that the adhesive bonding with the molten resin of low-density polyethylene and the like or a polyurethane-based adhesive as the adhesive layer must be preceded by anchoring treatment of the surface of the nonstretched polypropylene film as the heat-sealable film in order to improve the adhesion with the adhesive layer. A problem in the anchoring treatment, however, is that, because the treatment is performed by use of an organic solvent, the solvent more or less remains unavoidably in the laminated film after bonding with the base film to cause disadvantages in respect of safety and unpleasant odor in the foods wrapped with the film due to the migration of the solvent thereto. Moreover, increase in the production costs is unavoidable in the anchoring treatment including the investment for the treatment facilities, cost of energy consumed in the step of drying and cost for the materials used in the treatment and the anchoring treatment is also undesirable in respect of the workers' health and safety as well as in environmental pollution so that improvement has been eagerly desired.
Indeed, no heat-sealable film is available which in itself is excellent in the heat resistance, rigidity and other properties and can satisfy simultaneously the adhesive bondability to a base film or a low-density polyethylene for extrusion lamination and the heat-sealability in bag making as well as the strength of heat-sealing.